This invention relates generally to apparatus for supplying fuel to an engine and more particularly, but not by way of limitation, to auxiliary fuel injection apparatus for a turbocharged, spark-ignition engine.
The fuel required by an internal combustion engine can be estimated by the equation: EQU F=1/2V.sub.D .times.N.times..eta..sub.v .times..rho..sub.a .times.1/AF.times.60 (lbs. fuel/hr.) (1)
where:
______________________________________ V.sub.D = swept volume of the engine (ft..sup.3) N = engine speed (rev./min.) N.sub.v = volumetric efficiency (%) P.sub.a = air density (lbs. air/ft.sup.3) AF = mass ratio of air to fuel (lbs. air/lbs. fuel). ______________________________________
If .rho..sub.a =.rho..sub.as .times..rho..sub.R, where .rho..sub.as is the air density at standard temperature and pressure and .rho..sub.R is the density ratio of air density at a given pressure to .rho..sub.as, then equation (1) can be rewritten as: EQU F=1/2V.sub.D .times.N.times..eta..sub.v .times..rho..sub.as .times..rho..sub.R .times.1/AF.times.60 (lbs.fuel/hr.) (2)
For a naturally aspirated engine (i.e., one which is not supercharged) and neglecting temperature effects, it can be assumed that .rho..sub.R approximately equals 1. Further assuming that the mass ratio of air to fuel (AF) is constant at a wide open throttle setting of the engine and that the change in the volumetric efficiency (.eta..sub.v) is negligible over the engine speed range, then the only variable in the preceding equation is engine speed (N). Under these assumptions, equation (2) reduces to, for a naturally aspirated engine: EQU F.sub.NA .times.K.sub.NA .times.N, (3)
where EQU K.sub.NA =1/2V.sub.D .times..eta..sub.v .times..rho..sub.as .times.1/AF.times.60.
By setting N to its maximum value, one can determine the maximum fuel requirement of the engine under the foregoing assumptions. Knowing the maximum fuel requirements of the engine under naturally aspirated conditions, one also has an indication of the maximum fuel delivery capability of the engine's original or stock fuel system supplied by the manufacturer.
When the engine is supercharged, such as by a turbocharger for example, .rho..sub.R can increase to values significantly greater than unity due to the increase in pressure in the intake manifold of the engine. At wide open throttle with a properly designed turbocharger system, the intake manifold pressure (i.e., boost) will reach a maximum value at a relatively low engine speed. Therefore, for a turbocharged (or, more generally, supercharged) engine, .rho..sub.R can be assumed to be constant over most of the engine speed range. However, because .rho..sub.R is greater than unity, the total fuel requirement for a turbocharged engine is approximated by: EQU F.sub.TC =K.sub.NA .times.N.times..rho..sub.R ( 4)
By subtracting equation (3) from equation (4), the additional fuel required in a turbocharged engine over that required in a naturally aspirated engine is: EQU F.sub.A =K.sub.NA .times.N.times.(.rho..sub.R -1). (5)
Because most original manufacturers' fuel systems deliver fuel in proportion to detected air flow, it can be assumed that the fuel flow will be metered correctly by the original system until the air flow exceeds the maximum naturally aspirated air flow at maximum engine speed. Therefore, the additional fuel which needs to be added by a supplemental fuel supplying apparatus is indicated by the equation: EQU F.sub.A =K.sub.NA .vertline.N.rho..sub.R -N.sub.max .vertline.(6)
Under supercharged conditions, it is apparent from the foregoing that if additional fuel is not added to the engine, the air-fuel ratio will increase into the lean region which is potentially dangerous to the engine. Such a condition creates the potential for detonation which could destroy the engine. This condition arises from the inability of the original fuel system to supply the additional fuel needed under supercharged conditions.
To overcome the deficiency of the original manufacturer's fuel system and thus to reduce the potential for detonation there are three alternatives of which I am aware. The first alternative is to add water (or a water-methanol mixture) to the intake air charge in the engine. The water reduces the intake charge temperature and the water becomes steam in the combustion chamber, thereby reducing the potential for detonation and enabling the engine to be more safely operated at a higher boost and at a leaner air-fuel ratio. Unfortunately, water, even mixed with a combustible fluid like methanol, does little to increase the power output of the engine. Additionally, to add such a substance or substances to the intake air charge requires a separate, secondary fuel system (fluid reservoir, pump, etc.) which must be properly maintained for safe engine operation. This alternative also has a limited capability of preventing detonation.
The second alternative of which I am aware is to retard the ignition point in either stepped or continuous response to manifold pressure as known to the art. Modification of the ignition point may be accomplished with or without any addition of an anti-detonant fluid, such as fuel or water. It is to be noted, however, that such retardation of the ignition point can adversely affect the engine operation by reducing the power output because the combustible mixture has an insufficient time to completely burn. Additionally, if the combustible mixture has an insufficient time to burn in the combustion chamber, the mixture will still be burning as it leaves the combustion chamber, thereby exposing all the components in the exhaust system (e.g., the exhaust valves and the turbocharger turbine) to potentially destructive exhaust gas temperatures. Retardation of the ignition point also reduces the engine's efficiency as measured by the brake specific fuel consumption value which measures the engine's efficiency in fuel consumption per hour per horsepower. Therefore, the useful effect of this method is limited.
The third alternative of which I am aware is to add more of the primary fuel to the intake air charge. I am aware of prior systems which add fuel with an increase in boost either in step changes at one or more predetermined pressures or continuously in proportion to boost. Neither of these types of systems, however, provides the desired air-fuel ratio until the engine speed approaches its maximum. Therefore, the engines with which these systems are used give less than their maximum power outputs, and the engines consume an excessive amount of fuel (i.e., more fuel than is required for proper performance) at low and middle engine speeds. Additionally, in some of these prior systems the fuel is added by varying the pressure differential across a fixed orifice in proportion to boost, thereby making the degree of fuel atomization, and thus the homogeneity of the air-fuel charge, a function of boost. This relationship causes the air-fuel charge to be non-uniform which adversely affects (or at least fails to enhance) the operation of the engine.
Therefore, there is the need for an apparatus for supplying additional fuel to an engine when the capacity of the original manufacturer's fuel system accompanying the engine is incapable of meeting the fuel requirements for maintaining proper air-fuel ratios. There is also the need for this apparatus to increase the power output and fuel economy of the engine over power output and fuel consumptions of prior devices which have been designed or developed to add fuel to an engine. To meet this need of increased power output and fuel economy, there is the further need for the apparatus to continuously control the air-fuel ratio when the engine intake air pressure exceeds the pressure of the maximum naturally aspirated air flow at maximum engine speed. There is also the need for the additional fuel to be added so that the combustible air-fuel charges are homogeneous.